Generally, lamps include a vacuum glass tube and a filament installed in the glass tube. The lamps are typically classified into illumination lamps, which generate light when current flows in the filament, and heating lamps which generate heat in the filament. Such a lamp is manufactured by installing a filament in a vacuum glass tube and installing terminals on the opposite ends of the glass tube to connect the filament to the outside. In a detailed description, the lamp is manufactured by installing the tungsten filament in the glass tube along the axis thereof, injecting iodine gas in the glass tube, and sealing the glass tube. When electric current flows into (electricity is applied to) the filament of the lamp manufactured in this way, tungsten atoms present in the filament combine with iodine on the wall of the glass tube, thus being converted into tungsten iodide. Thereafter, the compound returns to the filament. The tungsten iodide returning to the filament is decomposed, so that tungsten remains in the filament. Such a process is called an iodine cycle. The lamp undergoing the iodine cycle can be used very efficiently for a lengthy period of time.
However, the conventional lamp operated as described above is problematic in that the filament may be easily damaged by external impacts, and the filament may be easily deformed due to generated heat. That is, the lamp is not durable. Further, the conventional lamp is problematic in that a high cost is required to install the filament, so that the lamp is expensive.
Meanwhile, carbon fibers used in a sheet-type heating element or the like form a bundle consisting of very fine carbon fibers. For example, assuming that 26,400 carbon fibers are prepared and each carbon fiber is 1 m in length and 0.3 mm in diameter, the bundle of carbon fibers has the resistance value of about 60Ω. Thus, the desired power (watt) is designed based on such a principle, thereby the sheet-type heating element is manufactured. In this case, the resistance value is determined according to the resistance equation: R=rho (l/s). In the equation, R denotes resistance, p denotes resistivity, l denotes length, and s denotes a unit area. However, the carbon fibers are used as a heating source of the sheet-type heating element, which is designed to generate a temperature ranging from about 50° C. to about 70° C. If the temperature exceeds 70° C., there is a danger of fire, and the sheet-type heating element may be oxidized by oxygen, so that the durability of the sheet-type heating element will be remarkably reduced.
Meanwhile, a heating lamp has been proposed, which uses the carbon fiber as a heating source and installs the carbon fiber in a vacuum tube. However, the technology of forming a certain bundle of carbon fibers to determine the resistance value and thus provide a desired power, the technology of securing carbon fibers to terminals, and the technology of bundling carbon fibers are below a desired level. Thereby, it is difficult to industrialize the heating lamp. As one example of the technology, a carbon-based heating element has been proposed, which is disclosed in Japanese Patent Laid-Open Publication No. 2000-123960. According to the cited document, as shown in FIG. 1, cap-shaped electrode parts 2 are provided on the opposite ends of a carbon-based heating element 1. The carbon-based heating element 1 and the cap-shaped electrode parts 2 are installed in a vacuum hermetic tube 3. The cap-shaped electrode parts 2 are connected to lead wires 4 for applying electricity. As shown in FIG. 2, each lead wire 4 is secured to a carbon core 5, which is formed by binding the outer circumference of a bundle of carbon fibers 6 with carbon yarns 7.
The heating element 1 comprises at least one carbon core 5, and the cap-shaped electrode parts 2 are mounted to the opposite ends of the heating element 1. The components combined in this way are housed in the vacuum hermetic tube 3.
In such a heating element, a desired carbon fiber 6 is selected and a desired number of carbon fiber bundles is used to provide a desired resistance value and thus output a desired power W. However, the heating element is problematic in that it is complicated to bind the carbon fibers 6 with the carbon yarns 7, and the carbon core must be impregnated into liquid resin to prevent the tied carbon yarns 7 from being removed, as necessary.
Meanwhile, in order to increase the power, a method of increasing the length of carbon fibers has been proposed, in place of increasing the number of carbon fibers. This is disclosed in Japanese Patent Laid-Open Publication No. 2002-63870 (US Patent Laid-Open Publication No. 2001/0055478A1), and is illustrated in FIG. 3. As shown in the drawing, lead wires 4 are provided on the opposite ends of a vacuum hermetic tube 3, and electrode pieces 4-1 connected to the lead wires 4 in such a way as to conduct electricity are seated on plane terminal parts 3-1 which press and support the opposite ends of the vacuum hermetic tube 3. Further, spacers 13 are installed at regular intervals so as to support a coil band-type carbon-fiber filament 10 on the inner wall of the vacuum hermetic tube 3. Support terminals 20 each having a power applying sleeve 20-1 are installed on the opposite ends of the carbon-fiber filament 10. Each of the support terminals 20 includes the sleeve 20-1, and a connecting piece 20-2 which is integrated with the sleeve 20-1 and is connected to an intermediate terminal 20-3.
However, such a technology functions to simply secure the carbon-fiber filament 10 to the intermediate terminals 20. The technology is problematic in that it is difficult to locate the filament 10 at a central position in the vacuum hermetic tube 3, so that the spacers 13 must also be installed. Further, the carbon-fiber filament 10 has a structure obtained by arranging the bundle of carbon fibers to a predetermined width and forming the bundle in a band shape. Thus, the coupling force between the carbon fibers is weak, so the carbon fibers constituting the carbon fiber bundle may be separated from each other by impact or after use for a lengthy period of time, and thereby durability may be reduced.
Meanwhile, an example of a heating lamp, which uses a carbon fiber strand obtained by twisting carbon fibers in the form of a band, as a heating element, is disclosed in U.S. Pat. No. 6,534,904. As shown in FIG. 4, the heating lamp is constructed so that a heating element 2a which is wound spirally and has the shape of a carbon ribbon is accommodated in a vacuum hermetic tube 3, and external electricity is supplied through support terminals 20 and connectors la to the opposite ends of the heating element 2a. In this case, the heating element 2a is constructed to have a length which is 1.5 times as long as the length B of the vacuum hermetic tube, thus providing a desired power. That is, the heating element 2a has a spiral shape such that the heating element extends to a predetermined length to have a desired resistance value. However, such a technology is problematic in that there is no component for supporting the heating element 2a, so that the heating element 2a may sag and come into contact with the inner wall of the hermetic tube 3. Due to such contact, overheating occurs, so durability is reduced, and thereby it is difficult to industrialize the heating lamp.
An apparatus for manufacturing a carbon-ribbon-type heating element was proposed in U.S. Pat. No. 6,464,918. Referring to FIG. 5, the apparatus includes a spiral shaft 4b, a feeding means 10b, a motor 12b, a hot air fan 5b, a nozzle 6b, and a drive motor 11b. The spiral shaft 4b has the same diameter as the heating element to be wound. The feeding means 10b feeds a carbon ribbon 3b into the spiral shaft 4b . The motor 12b provides a driving force to the feeding means 10b. The hot air fan 5b heats the carbon ribbon 3b which is fed through the feeding means 10b. The nozzle 6b discharges hot air through the hot air fan 5b to the carbon ribbon 3b. The drive motor 11b, coupled to the hot air fan 5b, moves the hot air fan 5b along a rail 7b in the direction of arrow 9b. In this case, the rail 7b is installed to be parallel to the spiral shaft 4b. Reference numeral 13b denotes a control line or an actuating means which drive the motors 11b and 12b simultaneously. Further, it is desirable that the carbon ribbon 3b have a tension force 8b so that the carbon ribbon 3b is wound around the spiral shaft 4b in a constant fashion. Subsequently, hot air of about 300° C. is supplied from the hot air fan 5b to soften the carbon ribbon. The feeding means 10b feeds the carbon ribbon 3b at the same speed as the moving speed of the hot air fan 5b, so that the carbon ribbon 3b spirally wound around the spiral shaft 4b is softened. When the carbon ribbon has been wound, it is heated at about 1000° C. in pressure of nitrogen gas, and thereafter is cooled, so that the simple carbon ribbon has a spiral shape, and thereby the heating element of FIG. 4 is obtained. Such a process changes the properties of the simply wound carbon ribbon to a spiral structure having a restoring force. That is, resin in the heating element comprising carbon fiber/resin constituting the carbon ribbon is evaporated at high heat (1000° C.), so that the heating element contains only carbon. Thereby, the properties of the heating element are changed to be hard (but the heating element is thin, and so has elastic force). Consequently, the spiral heating element is obtained.
However, such a heating element is based on a band-shaped heating element, so that it is limitedly able to maintain its elastic force, and it is difficult to produce the heating element as a product. Further, as shown in FIG. 3, the spacers must be installed at regular intervals, so that marketability is poor.